Not directly related to Downton Abbey, but here is a rather interesting article on the evolution of the British and American accents. The most interesting aspect is that it is the British accent that has changed the most since American Independence. From the article:
In 1776, whether you were declaring America independent from the crown or swearing your loyalty to King George III, your pronunciation would have been much the same. At that time, American and British accents hadn't yet diverged. What's surprising, though, is that Hollywood costume dramas get it all wrong: The Patriots and the Redcoats spoke with accents that were much closer to the contemporary American accent than to the Queen's English. It is the standard British accent that has drastically changed in the past two centuries, while the typical American accent has changed only subtly.
It goes on to say that it was the British upper class that prompted the change.